The injecting of water into the inlet of gas turbines is used for power enhancement and also for improving the efficiency. The power enhancement is brought about both by the reduction in the compression effort and by the increase in the mass flow.
The injecting of water into the compressor can have two physical effects:
1. Inlet cooling:
The evaporation of water in the inlet upstream of the first blade row of the compressor, which saturates the air flow, leads to a lowering of the compressor inlet temperature on account of the latent evaporation heat.
2. Wet Compression:
The evaporation of water in the compressor creates a phenomenon which is referred to as “wet compression” (see, for example, WO 03/048545). The compression of the resulting 2-phase flow entails the transfer of heat from the gas phase to the liquid phase for the evaporation, which cannot be considered as an adiabatic process. Furthermore, the resulting steam from the droplet evaporation has an influence upon the mass flow in the compressor passage and an increase in the specific heat Cp on account of the change in the composition of the gas phase.
Numerous attempts have already been made in practice in order to achieve the power enhancement, in some cases by means of inlet cooling by using evaporation coolers or atomization, and in other cases via wet compression by supersaturation of the inlet air flow, such as in the case of “high fogging” or, less frequently, by water injection between the stages of the compressor (“interstage water injection”).
“High fogging” systems can achieve a power enhancement by 7-8% with a water mass flow of 1%, in fact with comparatively low installation costs. There are numerous limits and disadvantages, however, which are associated with this application such as:
1. Limits of the Operation on Account of Environmental Conditions
                Operating limit at low ambient temperature on account of icing. Due to the flow acceleration downstream of the variable inlet guide vanes (VIGV) and the temperature drop which is associated therewith, operation at ambient temperature below 10° C. can be limited.        Air inlet cooling is involved for relatively dry ambient conditions (relative humidity <60%) on account of the disturbance in the temperature profile which is associated therewith.2. Erosion        The inlet-side blade rows are affected by erosion on account of the occurrent drops. This can have an influence upon the mechanical integrity of the blade airfoil and can lead to an increase in the risk of damage to the compressor and also have an influence upon the operating range on account of disturbances in the leading-edge geometry of the front blade row operating in the supersonic range. Possible countermeasures, such as laser coating with rare materials, are costly.3. Reduction in the Surge Limit Margin        On account of the changes in the volumetric flow en route through the compressor, individual stages operate beyond the designed range. In the inlet stages, a flow, now becoming greater, can lead to an increase in the meridional velocities and so alter the velocity triangles so that both the impingement and the deflection are reduced. An additional effect which reduces the deflection (deviation in the flow over a compressor profile) on the inlet blades is the increasing slip factor on account of the wet surface of the blade airfoil. On the other hand, there is a decreasing volumetric flow in the rear stages because the increase in the mass flow on account of the evaporation results in a corresponding increase in the pressure in the compressor plenum and, in conjunction therewith, a reduction in the meridional velocities and an increase in the impingement and the deflection. This mismatch in the compressor stages can lead to an increase in the losses and to an aerodynamic disadvantage with regard to the efficiency. Overall, a shift of the compressor load from the front stages to the rear stages occurs as a consequence. The load which is shifted to the rear stages and the fact that the compressor operates on a raised working characteristic line, reduces the vertical surge limit margin, especially in the case of lower aerodynamic velocities. A certain minimum surge limit margin, however, should be observed in relation to a low-frequency operation of some unstable networks. Therefore, for “high-fogging” machines either a greater rigidity (high solidity in compressor blading) in the rear stages of the compressor should be provided, which increases the equipment costs and can decrease efficiency, or tighter limits in the protective concept against an underfrequency should be set, which reduces the availability of the system.4. Reduction in the Cooling Air Supply in the Center Region of the Compressor        Pressure and temperature are influenced en route through the compressor as a result of the process of “wet compression”. Since the front stages are loaded less, lower pressures are achieved in the air bleed cavities in comparison to dry operation. Also, the temperatures in the air bleed cavities are lower on account of the gas-phase feed of evaporation heat to the liquid phase. With invariable geometry of the secondary air system, a reduced cooling-air mass flow to the turbine components results from this, which can influence the service life of the turbine components. An alleviation could be effected in this case by a variable, modulating geometry of the secondary air system, which, however, increases the costs and the complexity of the system. Or, the volume of injected water is limited, resulting in a reduction in the power enhancement.        
A device and a method for enhancing the output power of a gas turbine are known from printed publication EP 1 903 188 A2, in which provision is made for a washing unit which injects water into the gas turbine in order to remove deposits from the blades of the compressor. Furthermore, water can be injected into the air flow which is to be compressed in order to increase the mass flow and to augment the output of the turbine.
A combined cleaning and cooling device, with which water can be injected at different points of a compressor, is also known from U.S. Pat. No. 6,398,518.
From printed publication U.S. Pat. No. 6,634,165 it is known to control the injecting of water in the inlet tract of a gas turbine so that specific boundary conditions are maintained.
Known systems do not address a change of load in the individual compressor stages.